The present invention relates to a new and improved construction of an optical arrangement for an infrared intrusion detector which is of the type containing a number of optical focussing means which focus infrared radiation, arriving from a plurality of separate receiving regions or fields of view, upon at least one common sensor element.
Such arrangements take-up or receive the infrared radiation emitted by an individual at a monitored region and transmit such received infrared radiation to a sensor element. If the monitored region is divided into a number of separate receiving regions or fields of view between which there are located dark fields or zones, then each movement of a person causes a modulation of the infrared radiation received by the sensor element. This modulated infrared radiation can be evaluated by means of a conventional evaluation circuit for the purpose of indicating that an intruder has entered the monitored region or area and for giving an alarm signal.
In order to obtain the required separate receiving regions or fields of view it is known, for instance, from U.S. Pat. No. 3,703,718, U.S. Pat. No. 4,058,726 and U.S. Pat. No. 4,081,680, to provide a plurality of reflectors which are aligned in different directions. This plurality of reflectors focus the radiation arriving from different receiving regions upon the same sensor element. Each reflector is operatively correlated with a different receiving region and only focusses the radiation from such receiving region upon the sensor element. What is disadvantageous with this system design is that the entire receiving surface is divided into a great many small segments. Therefore, only a small quantity of radiation is taken-up or received from the individual receiving regions, so that the sensitivity of such infrared intrusion detectors frequently is inadequate, particularly if there are provided a great number of receiving regions or fields of view.
This shortcoming can be avoided with the arrangements disclosed in U.S. Pat. No. 3,760,399, U.S. Pat. No. 3,829,693 or U.S. Pat. No. 3,958,118, wherein there is provided a single reflector for all receiving regions or fields of view and the division into the individual regions is accomplished by means of a number of juxtapositioned or adjacently arranged sensor elements. While there is available a common, relatively large reflector surface or area for all receiving regions, nonetheless the plurality of sensor elements require a complicated and disturbance-prone circuit, and additionally, there is markedly limited the number of possible sensor elements, and thus, the receiving regions.
In British Pat. No. 2,012,045 and in the European Pat. No. 0 014 825 there have become known in this technology arrangements wherein the focussing of the infrared radiation upon a common sensor element is accomplished by multiple reflections. Here, the first reflection however again is accomplished at individual mirror or reflector segments, and each such mirror segment is correlated to a different receiving region or field of view. Therefore, such arrangements likewise are afflicted with the drawback that only a small quantity of radiation is received from the individual receiving regions and the sensitivity of the detector is thus frequently inadequate. In order to nonetheless obtain a good sensitivity, it was therefore necessary with such state-of-the-art infrared intrusion detectors to use relatively large mirror segments, so that the dimensions of such infrared intrusion detectors became relatively large. Hence, it was hardly possible to mount such relatively large sized detectors in a manner such that they were unnoticed or imperceptible, as the same is frequently desired for intrusion detection equipment.